Yes, that can work, and it’s a fairly common trick for long conduit runs.
The usual method is:
- Tie a lightweight piece of string (or thin nylon line) to a small wad of tissue paper, foam, or a plastic bag “parachute.”
- Place it at one end of the conduit.
- Use a vacuum cleaner at the other end to suck the wad through the conduit.
- Once the string is through, use it to pull a stronger draw cord.
- Use the draw cord to pull the CAT5 cable through.
For a 50 m conduit, suction is often more effective than trying to push a fish tape all the way through, especially if the conduit has several bends.
Some people use a vacuum cleaner to suck the line through; others use the blower outlet to blow a lightweight “mouse” through. Suction is generally easier to control and less likely to jam.
A few practical points:
- The conduit should be reasonably clear of water, mud, and obstructions.
- A lightweight braided nylon line works better than cotton string.
- Don’t pull the CAT5 directly with a very thin string; first pull through a stronger draw rope.
- If the conduit contains mains wiring, avoid adding network cable unless regulations and cable types permit it.
For a clean 50 m plastic conduit, it’s entirely plausible that your friend got a string through in a minute or two using a vacuum cleaner.